1. Field of the Invention
The invention relates to a method of producing a circular metal part which has a central hollow and is reinforced with fibers in the vicinity of this hollow, and is particularly concerned with the production of such parts for use in making rotors having a high strength/mass ratio, such as the rotors of turbo-machines used in aeronautics.
2. Summary of the Prior Art
Turbo-machine rotors are generally symmetrical about their axis of rotation and include, either on their periphery or on their side, a plurality of blades for operation in a compressor or a turbine. These rotors generally revolve at high speed, and are subjected to considerable stresses originating mainly from centrifugal force, but also from the vibrations of the machine and the accidental ingestion of foreign bodies.
The design of these rotors is a compromise between aerodynamic or hydrodynamic performance, strength, and mass. In order to improve the strength/mass ratio, it is known to reinforce such rotors by rings or bands made of stronger materials and having a greater modulus of elasticity, also termed Young's modulus.
Rotor reinforcement rings are known having a fibrous structure, especially fibers of silicon carbide, boron carbide, or other high strength material, wherein the fibers are embedded in a metallic matrix. French Patent 2607071 discloses an example in which silicon carbide (SIC) fibers are assembled into a helical strip or preform, and another strip, made of the material of the matrix, is interposed between the turns of the fiber strip. The assembly is pressed at high temperature so that the pressure forces diffuse the matrix material between the fibers to ensure the cohesion of the assembly.
European Patent 0490629 discloses another example in which the preforms are each made using a single fiber wound in a plane spiral. This process achieves a remarkable uniformity in the radius of curvature of the fiber, for an improved ring strength.
It must be noted, however, that these techniques require a pressing in an axial direction, which tends to cause undulation of the fibers and thus reduces their ability to withstand centrifugal stress.
Also known from French patent 2666262 is a process in which a composite band of fibers and metal matrix is spirally wound inside a metal ring, and then welded to it by isostatic or radial pressure while hot in an autoclave or with the aid of a thermal expansion mandrel.
The advantage of this process is that the reinforcement fibers are well stretched, which prevents any corrugation and improves their ability to withstand centrifugal stress.
However, all of these known processes suffer from the drawback of imposing two thermal cycles on the reinforcement fibers, namely compression of the fibers with the metal of the matrix to obtain the composite layers, then diffusion welding of the composite layers, which increases the degradation of the fibers. For example, silicon carbide SiC, which is often used for making reinforcement fibers, may react with the metal of the matrix at high temperature. To reduce this phenomenon, the fibers may be covered with a layer of carbon, but this layer itself diffuses into the metal of the matrix and alters the metallurgical structure of the latter. The reaction of the material of the fiber with the matrix is thus only delayed.